The present invention relates to an overvoltage protection circuit and is especially but not exclusively suited to use in a telephone line.
A telephone line from a telephone exchange to the subscriber's apparatus carries the voice signal and also a power supply to the subscriber's apparatus. Usually, the power supply consists of a d.c. voltage of about 40 volts. Hitherto, the voice signal has been an analogue signal representing the waveform of the voice. In future developments of telephone systems the analogue voice signal is to be replaced by a digital signal and in accordance with the ISDN (integrated services digital network) system the bit rate of the digital voice signal is 144 k bits/s and higher bit rates. Imbalance of the conductors of a telephone line is not troublesome for the analogue voice signal since the frequencies involved are less than 5 kHz and although some of the control signals transmitted down the line use frequencies up to 20 kHz no significant interference or other difficulties have been encountered. On the other hand, when high bit rate digital data is transmitted along a telephone line it is very important that the line is balanced so that radiation of the data signals and the picking up of interfering signals is avoided. To this end, there are specific requirements laid down in certain countries defining the permissible degree of imbalance between the conductors of a telephone line. In Germany, for instance, the requirement is that the balance should be better than -66 dB (1 in 2000).
Overvoltage protectors are commonly provided in telephone lines to limit the voltage on the line relative to ground and differential voltage between the conductors of the line, the requirement for such protectors being made more important by the increasing use of semiconductor circuitry in telephone apparatus in place of the hybrid transformers and other similarly robust but bulky components.
FIG. 1 as described below shows an example of a conventional overvoltage protection circuit connected to a telephone line.
In FIG. 1, data to be carried by the telephone line from an exchange is applied to a winding 1 of a transformer 2 having two similar windings 3 and 4 coupled to the winding 1. The telephone line itself consists of a conductor 5 and a conductor 6 connected to the windings 3 and 4 respectively, the other ends of which windings are connected through a filter 7 to conductors 8 and 9 to which supply voltages of 0 volts and -40 volts are respectively applied. In the subscriber's apparatus the conductors 5 and 6 are respectively connected to windings 10 and 11 of a transformer 12 which has a further winding 13 connected to the subscriber's handset. The other ends of the windings 10 and 11 are connected through a filter 14 to conductors 15 and 16 where the 0 volt and -40 volt levels are produced for use in powering other circuitry int he subscriber's apparatus (not shown).
In order to protect the circuitry connected to the conductors 5 and 6 against excessively high voltages applied to those conductors, for example as a result of a lightning strike, overvoltage protectors P1 and P2 are provided connected in series, with the junction point 17 of the two protectors connected to ground. The protectors P1 and P2 provide overvoltage protection against an excessively high voltage differential between the conductors 5 and 6, and also against an excessively high voltage relative to ground on either of the conductors 5 and 6.
Commonly used types of voltage protector include reverse biassed PN junctions, so that in such a protector when in use a depletion layer is set up at the junction, the thickness of which layer will depend upon the voltage applied across the protector. If the voltage across the protector is close to 0 volts, then the thickness of the depletion layer will be small with the result that the protector will have a high capacitance. On the other hand, if a relatively high voltage, but not one sufficient to cause the protector to conduct, were to be applied to the protector, the depletion layer would be thicker and consequently the capacitance presented by the protector would be relatively small. The conductor 5, which is maintained at 0 volts by the exchange equipment, has, at the point where the protector P1 is connected to it, a voltage close to that of ground, differing from ground potential only as a result of the voltage drop along the conductor due to current in it. Therefore the protector P1 will present a relatively high capacitance Cl, shown in dotted form in FIG. 1. The conductor 6, on the other hand, is at a potential close to -40 volts so that the protector P2 has a relatively smaller capacitance C2. The difference between the values of capacitances C1 and C2 leads to imbalance of the conductors 5 and 6 of the telephone line. Whilst this imbalance would, as mentioned above, be acceptable when the telephone line is carrying relatively low frequency signals (less than 20 kHz), it would not be acceptable when the line is carrying digital signals of 144 kilobits/s or of a higher rate.
Alternative forms of overvoltage protectors such as gas discharge tubes have a low capacitance, but whilst they could be used in the application described above, they would not provide such effective overvoltage protection as semiconductor devices using reverse biassed PN junctions.
The imbalance between the capacitances presented by the protectors P1 and P2 could be corrected by connecting a trimmer capacitance in parallel with the protector P2 (that presenting the lower capacitance), but this solution to the problem has three disadvantages. The first disadvantage is that the balance of the conductors of the line is strongly influenced by the value of the d.c. supply voltage, and it also varies with temperature which means that accurate balance cannot be maintained. A second disadvantage is that the trimmer capacitance would need to be adjusted separately for each individual protection circuit. Thirdly, the specification for the telephone system may require that a power failure produces the reversal of the supply voltages on the line and that would completely invalidate any fixed compensation of this kind.